First real time 7 Be solar detection in Borexino Davide D’Angelo INFN Sez. Milano On behalf of the Borexino Collaboration

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Contents Physics goals, detector design Detection principles and ν signature Detector design Radiopurity issues The last months Detector response, data analysis Event selection Detector response Background content Energy calibration and stability 11 C and neutrons after muons Spectral fits Comments on errors & Conclusions

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Borexino Collaboration Kurchatov Institute (Russia) Dubna JINR (Russia) Heidelberg (Germany) Munich (Germany) Jagiellonian U. Cracow (Poland) Perugia Genova APC Paris Milano Princeton University Virginia Tech. University

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Detection principles and signature Typical rate (SSM+LMA+Borexino) previous real-time measurements (SNO, SuperK) < 1/ of the total solar ν flux Borexino principal aim: mono-energetic MeV 7 Be Borexino threshold elastic scattering on electrons in highly purified liquid scintillator Only 7 Be ν considered so far. pep ν, CNO ν and possibly pp ν will be studied in the future Detection via scintillation light: + Very low energy threshold + Good position reconstruction + Good energy resolution BUT… – No direction measurement – The ν induced events can’t be distinguished from other β events due to natural radioactivity Extreme radiopurity of the scintillator is a must!

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Abruzzo, Italy 120 Km from Rome Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e Borexino Detector and Plants External Labs

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Detector layout and main features Water Tank: γ and n shield μ water Č detector 208 PMTs in water 2100 m 3 20 legs Carbon steel plates Scintillator: 270 t PC+PPO (1.4 g/l) Stainless Steel Sphere: ● 2212 PMTs ● ~ 1000 m 3 buffer of pc+dmp (light queched) Nylon vessels: (125 μm thick) Inner: 4.25 m Outer: 5.50 m (radon barrier)

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Picture gallery 2000 Pmt sealing: PC & Water proof 2002 PMT installation in SSS Nylon vessels installation (2004)

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Latest time schedule.... filling operations purging of the SSS volume with LAKN (early ‘06) water filling (Aug. 06  Nov. 06) replacement of water with PC+PPO or PC+DMP (Jan. 07  May. 07) DATA TAKING with fully filled detector from May 15, 2007

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Background summary table

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano What’s important of previous table… 238 U and 232 Th content in the scintillator and in the nylon vessels meet specifications or sometimes are even below specs GOAL: < g/g (< 10 c/d/FV)ACHIEVED: < g/g 14 C/ 12 C is ~ as expected ( measured) Muon rejection is fine: < Two main backgrounds are still above specs, although managable: Off equilibrium 210 Po  s (no evidence of 210 Pb or 210 Bi at that level) Some 85 Kr contamination: a small air leak during filling?

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Our first result (astro-ph v2) We have detected the scattering rate of 7 Be solar s on electrons: 7 Be Rate: 47 ± 7 STAT ± 12 SYS c/d/100 t 2 approaches... 1 result

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano The starting point: no cut spectrum 14 C dominates below 200 keV 210 Po NOT in eq. with 210 Pb Mainly external  s and  s Photoelectrons Statistics of this plot: ~ 1 day Arbitrary units

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano  rejection  are identified by OD and ID OD efficiency: > 99% ID analysis based on pulse shape: Deutsch variable: fraction of light in the PMTs with concentrators Pulse mean time & time of the peak. Overall rejection factor: > 10 4 (still preliminary)  are efficiently tagged for 7 Be residual background: < 1 c/d/100 t ID efficiency A muon in OD No cuts After  cut A muon in OD

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Position reconstruction and fiducial volume Position reconstruction algorythms (4 codes) Time-Of-Flight fit to hit time distribution developed with MC, tested and validated in CTF checked and tuned with 214 Bi-Po and 14 C events External background is large at the periphery of the IV  from materials (SSS, PMTs, cones) that penetrate the buffer They are removed by a fiducial volume cut: R < m (100 t) Additionally z < 1.8 m to remove some Rn events introduced during filling Radial distribution R2R2 gauss z vs R c scatter plot FV 214 BiPo (~800 keV) 14±2 14 C (~100 keV) 41±4 cm

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano 238 U and 232 Th content 212 Bi 212 Po 208 Pb   = ns 2.25 MeV ~800 keV eq. Only 3 bulk candidates 232 Th Events are mainly in the south vessel surface (probably particulate) 214 Bi- 214 Po 212 Bi- 212 Po 214 Bi 214 Po 210 Pb   = 236  s 3.2 MeV ~700 keV eq. 238 U: < g/g 232 Th: < g/g Assuming secular equilibrium, 232 Th and 238 U are measured with the delayed concidences:

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano 11 C and neutrons after muons  s may produce 11 C by spallation on 12 C n are also produced ~ 95% of the times Only the first neutron after a muon can be currently detected Work in progress to try to improve this Events that occur within 2 ms after a  are rejected Neutron Capture Time  ~ 210  s Neutron spatial distribution

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Final spectrum after all cuts  Kr+  Be  shoulder 14 C 210 Po (only, not in eq. with 210 Pb!) 11 C Understanding the final spectrum: main components Last cut: 214 Bi- 214 Po and Rn daughters removal No  s After fiducial volume cut (“100 tons”)

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Energy calibration and stability We have not calibrated with inserted sources (yet) Planned for the near (?) future So far, energy calibration determined from 14 C end point spectrum Energy stability and resolution monitored with 210 Po  peak Difficult to obtain a very precise calibration because: 14 C intrinsic spectrum and electron quenching factor poorly known Light yield monitored with 210 Po peak position Light yield determined from 14 C fit

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano 7 Be signal: fit without  subtraction Strategy: Fit the shoulder region only Use between 14 C end point and 210 Po peak to limit 85 Kr content pep neutrinos fixed at SSM- LMA value Fit components: 7 Be 85 Kr CNO+ 210 Bi combined very similar in this limited energy region Light yield left free 7 Be 85 Kr CNO Bi 210 Po peak not included in this fit These bins used to limit 85 Kr content in fit “only 85 Kr” and “no 7 Be” fit excluded by > 5 

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano  /  discrimination  particles Small deformation due to average SSS light reflectivity  particles pe; near the 210 Po peak pe; low energy side of the 210 Po peak 2 gaussians fit Full separation at high energy ns  Gatti parameter

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano 7 Be signal: fit  subtraction of 210 Po peak The large 210 Po background is subtracted in the following way: For each energy bin, a fit to the  Gatti variable is done with two gaussians From the fit result, the number of  particles in that bin is determined This number is subtracted The resulting spectrum is fitted in the energy range between 270 and 800 KeV A small 210 Po residual background is allowed in the fit 2 gaussians fit   The two analysis yield fully compatible results

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Comments on errors statistical: right now, it includes combined the effect of statistics itself, the lack of knowledge of 85 Kr content, and the lack of a precise energy calibration these components are left free in the final fit, and contribute to the statistical error systematic: mostly due to the fiducial volume determination with 45 days of data taking, and without an internal source calibration, we estimate an upper limit of 25% for this error can be much improved even without internal calibration with more statistics and better understanding of the detector response 7 Be Rate: 47 ± 7 STAT ± 12 SYS c/d/100 t

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Conclusions Borexino has performed the first real-time detection of sub/MeV solar neutrinos with just 2 months of data a clear 7 Be signal is visible after a few cuts better results to come in the near future (+ checks on day/night, seasonal or long term effects) the central value is well in agreement with MSW/LMA theoretical prediction with oscillations 49 ± 4 counts/day/100t measured rate 47 ± 7 stat ± 12 syst counts/day/100t no oscillation expectation 75 ± 4 counts/day/100t future scientific plans pp, pep and CNO neutrinos fluxes antineutrinos (earth, reactors, Sun) supernova neutrino magnetic moment

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Extra stuff just for questions...

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano 15 years of work Detector & Plants All materials carefully and painfully selected for: Low intrinsic radioactivity Low Rn emanation Good behaviour in contact with PC Pipes, vessels, plants: electropolished, cleaned with detergent(s), pickled and passivated with acids, rinsed with ultra-pure water down to class The whole plant is vacuum tight Leak requirements < atm/cc/s Critical regions (pumps, valves, big flanges, small failures) with additional nitrogen blanketing PMTs (2212) Sealing: PC and water tolerant Low radioactivity glass Light cones (Al) for uniform light collection in fiducial volume Time jitter: 1.1 ns (for good spatial resolution, mu-metal shielding) 384 PMTs with no cones for  id Nylon vessels Material selection for chemical & mechanical strength Low radioactivity to get <1 c/d/100 t in FV Construction in low 222 Rn clean room Never exposed to air

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano A matter of cleanliness Water ( production rate 1.8 m 3 /h) RO, CDI, filters, N 2 stripping U, Th: < g/g 222 Rn: ~ 1 mBq/m Ra: <0.8 mBq/m M  /cm 20°C Scintillator IV: PC+PPO (1.5 g/l) OV & Buffer: PC+DMP (5 g/l) PC Distillation (all PC) 6 stages distillation 80 mbar, 90 °C Vacuum stripping with low Ar-Kr N 2 222Rn: 8  Bq/m3 Ar: 0.01 ppm Kr: 0.02 ppt Humidified with water vapor 60-70% PPO purification PPO is solid. A concentrated solution (120 g/l) in PC is done first (“master solution”) Master solution was purified with: Water extraction ( 4 cycles) Filtration Single step distillation N 2 stripping with LAKN Filling operations Purging of the SSS volume with LAKN (early ‘06) Water filling (Aug. 06  Nov. 06) Replacement of water with PC+PPO or PC+DMP (Jan. 07  May. 07) Mixing online

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano The Problem of Polonium Po (alpha emitter) is present in Borexino at about 60 ev/(day x ton) [more than 100x the predicted 7 Be event rate] Immediate predecessor, the beta-emitter 210 Bi, is present (if at all) is less than 1/100 th Po contaminants may have complicated chemistry not as easily removed as Pb or Bi? Matches experience of other experiments (e.g. KamLAND, cf. Kishimoto talk at TAUP 2007) We see that 210 Bi, 210 Po are out of equilibrium Fitted 210 Po alpha peak Data after fid’l vol. and Rn cuts 210 Bi spectrum if it were in equilibrium with 210 Po > 2 orders of magnitude Fortunately, unsupported 210 Po goes away quickly! (  ~ 200 days) Will be much better in 1-2 years. Meanwhile, use  /  discrimination…

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Excluding Radon Daughters 214 Bi/Po are taggable by their coincidence and so can be easily removed from data sample. But also, note the progression of mean lifetimes in the radon decay chain: 222 Rn  218 Po  214 Pb  214 Bi  214 Po  = 4.4 min 39 min 28 min 237  s Excluding events preceding a 214 BiPo coincidence by 3 hours or less, and within 1 m of the 214 BiPo events’ spatial locations, lets us eliminate > 90% of each of these five species from the data, with little sample loss! This is particularly useful for the  -emitting isotope 214 Pb, whose spectrum has a broad peak near the 7 Be neutrino shoulder energy.

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano What Next: pep/CNO neutrinos Much harder targets to hit: rates are lower than 7 Be by another factor of 5-10 spectra mostly obscured by 7 Be neutrino signal above the 7 Be shoulder, partially obscured by cosmogenic 11 C (a β + -emitter) But also very scientifically desirable: pep rate is closely tied to that of the pp neutrinos that are obscured by 14 C CNO rate has great theoretical uncertainty (30%) depending upon unknown factors of the solar chemical composition And already we have a plan to reduce background: When a muon produces a 11 C atom, 95% of the time a neutron is released. The neutron is quickly (~200  s) captured by a proton, releasing a 2.2 MeV  11 C has a mean life of 29 min. The muon, neutron and 11 C decay can be correlated by their small spatial and temporal separations.

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano What Next: Geoneutrinos Anti-neutrinos are produced in Earth’s crust by radioactive  decay (of exactly the isotopes that cause problems for us in the detector!) We can see them via p + e  n + e + : first we see the positron annihilation (  1.02 MeV) then, with a mean life of ~ 200  s, we see the neutron capture 2.2 MeV  The reaction has a energy threshold of m n + m e - m p, or 1.8 MeV Expected rate is ~ 10 events/year in 280 tons of scintillator (with a background of reactor anti-neutrinos on the same order due to European nuclear reactors). But the amount of radioactivity in the Earth’s crust is not yet very well known, so this data will be welcome!

NNN Hamamatsu, Oct D. D’Angelo – INFN sez. Milano Supernova signals Galactic type-II SN of 3x10 53 erg at 10kpc + ~80ev of proton elastic scattering (quenching?)